instance: Add experimental checkpoint endpointv0.3.3

This commit adds an experimental endpoint that serves the log's to-sign
tree head formatted as a signed checkpoint.  See documentation at:

https://github.com/transparency-dev/formats/tree/main/log#readme

If we decide to adopt this endpoint we should consider in more detail
what the origin string should be.  Right now, it is assumed that the
log's configured prefix is unique across the set of all sigsum logs.

Example output:
```
$ echo "Public verification key: $vk"
Public verification key: 8cf3ac85aadd42891c5ae9aef27244cb2a546a2312f80020aad3f2ae1af73314
$
$ echo "Formatted as a note key: $vkNote"
Formatted as a note key: sigsum.org:testonly+8de2c54b+AYzzrIWq3UKJHFrprvJyRMsqVGojEvgAIKrT8q4a9zMU
$
$ curl http://localhost:6965/testonly/sigsum/v0/get-checkpoint
sigsum.org:testonly
23
HSt6W8ve4/36xAIf04qDOqaKLaKqSOUqKxLNrkK74+g=

— sigsum.org:testonly jeLFS4WQDEeTavbiYArHzCQUPXbQ1Y/V8/dKJlBxqOAY4eLhSh3uWNJ8YXp1vs/zR4SDNHI+6UybLmHmFlg/VSf5OAs=
```

1 files changed, 1 insertions, 0 deletions

diff --git a/pkg/instance/instance.go b/pkg/instance/instance.go
index bda553d..4dff31a 100644
--- a/pkg/instance/instance.go
+++ b/pkg/instance/instance.go
@@ -55,6 +55,7 @@ func (i *Instance) Handlers() []Handler {
 		Handler{Instance: i, Handler: getTreeHeadLatest, Endpoint: types.EndpointGetTreeHeadLatest, Method: http.MethodGet},
 		Handler{Instance: i, Handler: getTreeHeadToSign, Endpoint: types.EndpointGetTreeHeadToSign, Method: http.MethodGet},
 		Handler{Instance: i, Handler: getTreeHeadCosigned, Endpoint: types.EndpointGetTreeHeadCosigned, Method: http.MethodGet},
+		Handler{Instance: i, Handler: getCheckpoint, Endpoint: types.Endpoint("get-checkpoint"), Method: http.MethodGet},
 		Handler{Instance: i, Handler: getConsistencyProof, Endpoint: types.EndpointGetConsistencyProof, Method: http.MethodPost},
 		Handler{Instance: i, Handler: getInclusionProof, Endpoint: types.EndpointGetInclusionProof, Method: http.MethodPost},
 		Handler{Instance: i, Handler: getLeaves, Endpoint: types.EndpointGetLeaves, Method: http.MethodPost},